1. Field of the Invention
The invention relates to a variable gain amplifier and, in particular, to a linear-in-dB variable gain amplifier.
2. Description of the Related Art
FIG. 1 is a characteristic diagram of a linear-in-dB variable gain amplifier (VGA). In FIG. 1, a vertical axis represents a gain of the linear-in-dB VGA in dB, and a horizontal axis represents a control voltage of the linear-in-dB VGA in Volt. The gain of the linear-in-dB VGA can be expressed as Gain=ƒ(Vctrl)=A0·exp(K·Vctrl). Typical applications of the linear-in-dB VGA are automatic gain control (AGC) and power control in wireless transceivers, automatic gain control (AGC) in hard disk read channel ICs, and analog adaptive filter/equalizer used in high speed cable/optical receivers, partial-response most-likelihood (PRML) read channel ICs and pulse based ultra wide band (UWB) receivers.
Since the current to voltage relationship of a bipolar device is exponential, it is not difficult to design a linear-in-dB VGA using a bipolar process. However, current of an MOS device is a square function of a bias voltage thereof. As a result, it is difficult to design a linear-in-dB VGA using a CMOS process.